Detect against invalid variant index for LibStdC++ std::variant data formatters (#69253)

https://github.com/llvm/llvm-project/pull/68012/files

 added new data
formatters for LibStdC++ std::variant.

However, this formatter can crash if std::variant's index field has
invalid value (exceeds the number of template arguments).
This can happen if the current IP stops at a place std::variant is not
initialized yet.

This patch fixes the crash by ensuring the index is a valid value.

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Co-authored-by: jeffreytan81 <jeffreytan@fb.com>